Natural gas from many gas fields, which is often produced at high pressures, possibly as high as 50 MPa, can contain significant levels of H2O, H2S, CO2, N2, mercaptans, and/or heavy hydrocarbons that have to be removed to various degrees before the gas can be transported to market. It is preferred that as much of the acid gases H2S and CO2 be removed from natural gas as possible to leave methane as the recovered component. Small increases in recovery of this light component can result in significant improvements in process economics and also serve to prevent unwanted resource loss, It is desirable to recover more than 80 vol %, preferably more than 90 vol %, of the methane when detrimental impurities are removed. In many instances effective removal of the H2S is more important than CO2 removal as specifications for natural gas transmission pipelines typically limit the H2S content to be as low as 4 vppm while a more relaxed specification of two to three percent is typically permissible for CO2. If the contaminant removal process is unselective between these two gases or favorable to CO2 removal, the treatment will be unnecessarily severe, resulting in increased processing costs. A natural gas treatment process which is selective for H2S relative to CO2 is therefore economically attractive.
Natural gas treating is often carried out using solid sorbents such as activated charcoal, silica gel, activated alumina, or various zeolites. The well-established pressure swing adsorption (PSA) process has been used in this way since about the 1960s. In the PSA process, the solid sorbent is contained in a vessel and adsorbs the contaminant gas species at high pressure and When the design sorption capacity of the sorbent is attained the gas stream is switched to another sorption vessel while the pressure in the first vessel is reduced to desorb the adsorbent component. A stripping step with inert (non-reactive) as or with treated product gas may then follow before the vessel is returned to the sorption portion of the cycle. Variants of the conventional PSA (cPSA) process have been developed including the partial pressure swing or displacement purge adsorption (PPSA), rapid cycle pressure swing adsorption (RCPSA), Dual Bed (or Duplex) PSA Process, and rapid cycle partial pressure swing or displacement purge adsorption (RCPPSA) technologies.
Temperature swing adsorption (TSA) provides an alternative to the pressure swing technology in which the sorbed component is desorbed by an increase in temperature typically achieved by the admission of high temperature gas, e.g., air, to the vessel in the regeneration phase, Rapid cycle thermal swing adsorption (RCTSA) is a variant of the conventional TSA process using short cycles, typically less than two minutes. TSA processes are generally available commercially from a number of technology suppliers, although the state of the art for large scale rapid cycle TSA units is considerably less advanced. Large scale slow (˜10 hr) cycle internally heated TSA's have been used in natural gas processing for rigorous dehydration and mercaptan removal. In an internally heated thermal swing adsorption process, the gas or fluid used to heat the contactor directly contacts the adsorbent material. As such, the gas or fluid used to heat the contactor during regeneration can pass through the same channels that the feed gas does during the adsorption step. Externally heated thermal swing adsorption processes employ contactors having a separate set of channels to carry gases or fluids used to heat and cool the contactor so that gases used to heat and cool the contactor do not mix with the adsorbent that contacts the feed gas.
Indeed, adsorptive separation may be achieved, as noted by Yang by three mechanisms, steric, equilibrium, or kinetic: R. T. Yang Gas Separation by Adsorption Processes, Imperial College Press, 1997, ISBN: 1860940471, ISBN-13: 9781860940477. A large majority of processes operate through the equilibrium adsorption of the gas mixture and kinetic separations have lately attracted considerable attention with the development of functional microporous adsorbents and efficient modeling tools. Relatively few steric separation processes have been commercialized. Kinetically based separation involves differences in the diffusion rates of different components of the gas mixture and allows different species to be separated regardless of similar equilibrium adsorption parameters. Kinetic separations utilize molecular sieves as the adsorbent since they exhibit a distribution of pore sizes which allow the different gaseous species to diffuse into the adsorbent at different rates while avoiding exclusion of any component of the mixture. Kinetic separations can be used for the separation of industrial gases, for example, for the separation of nitrogen from air and argon from other gases. In the case of the nitrogen/oxygen separation (for example, oxygen and nitrogen differ in size by only 0.02 nm), the separation is efficient since the rate of transport of oxygen into the carbon sieve pore structure is markedly higher than that of nitrogen. Hence, the kinetic separation works, even though the equilibrium loading levels of oxygen and nitrogen are virtually identical.
Kinetically based separation processes may be operated, as noted in U.S. Patent Application Publication No. 2008/0282884, as pressure swing adsorption (PSA), temperature swing adsorption (TSA), partial pressure swing or displacement purge adsorption (PPSA) or as hybrid processes comprised of components of several of these processes. These swing adsorption processes can be conducted with rapid cycles, in which case they are referred to as rapid cycle thermal swing adsorption (RCTSA), rapid cycle pressure swing adsorption (RCPSA), and rapid cycle partial pressure swing or displacement purge adsorption (RCPPSA) technologies, with the term “swing adsorption” taken to include all of these processes and combinations of them.
In the case of kinetic-controlled PSA processes, the adsorption and desorption are more typically caused by cyclic pressure variation, whereas in the case of TSA, PPSA and hybrid processes, adsorption and desorption may be caused by cyclic variations in temperature, partial pressure, or combinations of pressure, temperature and partial pressure, respectively. In the exemplary case of PSA, kinetic-controlled selectivity may be determined primarily by micropore mass transfer resistance (e.g., diffusion within adsorbent particles or crystals) and/or by surface resistance (e.g., narrowed micropore entrances). For successful operation of the process, a relatively and usefully large working uptake (e.g., the amount adsorbed and desorbed during each cycle) of the first component and a relatively small working uptake of the second component may preferably be achieved. Hence, the kinetic-controlled PSA process requires operation at a suitable cyclic frequency, balancing the avoidance of excessively high cycle frequency where the first component cannot achieve a useful working uptake with excessively low frequency where both components approach equilibrium adsorption values.
Some established kinetic-controlled PSA processes use carbon molecular sieve adsorbents, e.g., for air separation with oxygen comprising the first more-adsorbed component and nitrogen the second less adsorbed component. Another example of kinetic-controlled PSA is the separation of nitrogen as the first component from methane as the second component, which may be performed over carbon molecular sieve adsorbents or more recently as a hybrid kinetic/equilibrium PSA separation (principally kinetically based, but requiring thermal regeneration periodically due to partial equilibrium adsorption of methane on the adsorbent material) over titanosilicate based adsorbents such as ETS-4 (such as disclosed in U.S. Pat. Nos. 6,197,092 and 6,315,817).
The faster the beds perform the steps required to complete a cycle, the smaller the beds can be when used to process a given hourly feed gas flow. Several other approaches to reducing cycle time in PSA processes have emerged which use rotary valve technologies as disclosed in U.S. Pat. Nos. 4,801,308; 4,816,121; 4,968,329; 5,082,473; 5,256,172; 6,051,050; 6,063,161; 6,406,523; 6,629,525; 6,651,658; and 6,691,702. A parallel channel (or parallel passage) contactor with a structured adsorbent may be used to allow for efficient mass transfer in these rapid cycle pressure swing adsorption processes. Approaches to constructing parallel passage contactors with structured adsorbents have been disclosed such as in U.S. Patent Application Publication No. 2008/0282892.
Traditionally, adsorptive separation processes use packed beds of adsorbent particulates. However, the traditional packed beds are not likely to meet the very stringent requirements for natural gas cleanup. The use of adsorbent monoliths provides one approach to designing an adsorbent bed that has low pressure drop, good flow distribution, and low dispersion. Monoliths have very low flow tortuosity and can also be engineered for almost any user specified void volume to meet a specified pressure drop. Other monolith advantages include avoidance of bed fluidization or lifting. While offering these advantages, the monoliths can also have some disadvantages. These include, (i) lack of lateral flow communication between axial flow channels which prevents self correction of any flow maldistribution, (ii) a likely more pronounced effect of obstructive fouling on flow distribution, (iii) potential thermal and mechanical stresses during pressure and thermal cycling, (iv) wall effects leading to flow leakage near the wall, (v) difficult and expensive to manufacture, (vi) difficult to apply a consistent and mechanically stable adsorbent coating within the monolith channels, and (vii) difficult loading/unloading of the monolith in the containment vessel (as compared to loose particle beds) leading to a longer turnaround time.
Other gas streams containing similar contaminants are encountered in various industrial processes, notably in petroleum refining and in petrochemical processes. In petroleum refining, for example, hydrodesulfurization processes utilize separation processes which remove the hydrogen sulfide formed in the process from the circulating stream of hydrogen. Conventionally, amine scrubbers are used for this purpose, using liquid amine sorbents such as monoethanolamine (MBA), diethanolamine (DEA), triethanolamine (TEA), methyldiethanolamine (MDEA), and diisopropylamine (DTPA) in the form of an aqueous solution.
Conventionally, liquid sorbent systems such as used in hydrogen sulfide scrubbing operate on a closed cycle with separate sorption and regeneration vessels through which the liquid sorbent is continuously circulated in a sorption-regeneration loop in which the sorption is typically carried out at a temperature optimized for sorption of the contaminant and the regeneration carried out by stripping, usually by steam at a higher temperature, in the regeneration tower. Inert gas stripping is also potentially useful to remove the sorbed contaminant species.
The capture of CO2 by amine species takes place through the formation of carbamate salts for primary and secondary amines, and additionally through the formation of ammonium bicarbonate salts when water is present. When tertiary amities are utilized with water present the formation of carbamate salts which require a proton transfer cannot take place and the reactions are limited to the formation of bicarbonate salts in a reaction sequence which with requires H2O to be present. In the absence of water, tertiary and other non-protogenic basic nitrogen species do not react with CO2, as no bicarbonate formation is possible. Hydrogen sulfide (H2S) is a Brønsted acid, and it reacts with all sufficiently basic amine species, including tertiary or non-protogenic amines, amidines, guanidines, and biguanides through simple acid/base reactions by the transfer of a proton from the H2S to the amine species to form ammonium sulfide (trisubstituted ammonium sulfide salts in the case of tertiary amines) reversibly, both in the presence and absence of water.
Current commercial RCPSA machinery can typically use a rotary module with two interfaces that bound the two ends of substantially parallel-sided adsorbent forms (e.g., tubes/cylinders), in which the interfaces have roughly the same cross-sectional area and shape. Such rotary modules typically occupy a relatively large volume, which can be a problem for volume-limited applications, such as floating- and/or platform-based separation/sequestration of carbon dioxide from natural gas. Nevertheless, typically only about 5-8 million standard cubic feet per day (MSCFD) of gas feed flow can be processed in a single commercial rotary module having a rotor diameter and/or sorption material diameter of approximately 3.5-4 feet (about 1 meter). Therefore, for swing applications processes requiring treatment of higher flows and/or for purification into more concentrated products from more dilute feedstreams, e.g., about 100-200 MSCFD or even higher, a very large number of these commercial modules would need to be utilized and integrated. When integrating the number of straight-walled adsorbent forms necessary for such high flow/throughput applications, volume limitations can often make such integration extremely difficult, or maybe impossible. While the volume-limited, high-throughput issue has been illustrated using a pressure swing example, it should be understood that temperature swing and/or mixed pressure and temperature swing processes should generally experience similar issues.
Alternately, instead of integrating a relatively large number of lower-capacity commercial rotary modules to attain higher gas throughput, one might simply scale up a single rotary module to achieve the 12- to 40- fold (or higher) increase in capacity. Because adsorption capacity typically scale with volume, a doubling in apparatus (valve) size can ideally effect an 8-fold increase in capacity, though this scaling does not necessarily apply to flow rates/fluxes, As the commercial rotary module valve diameters are already typically about four feet, the mechanical aspects of increasing diameter by 100% (or more) may not be trivial and may cause additional, previously-unrecognized issues.
Further, parallel-sided adsorbent modules may not offer the best per gram adsorptive efficiency, due to the lower need for sorption volume at the product end of the module as compared to the feed end. In such a situation, it may be desirable to seek to increase the per gram adsorptive efficiency of the adsorbent material by altering the shape of the adsorbent module(s), such that the cross-sectional area at the product end is smaller than the cross-sectional area at the feed end of the module(s), and/or such that the adsorbent module(s each) has(have) sides that are not parallel but that are at an angle so as to converge/intersect (though generally the convergence/intersection would be at a theoretical position past the product end of the module(s)). Additionally or alternately, in situations where rotary valved non-parallel-sided adsorbent modules
There is, therefore, a need to increase capacity, or at least the efficiency, of each single swing adsorption module to reduce cost, to make the technology applicable to larger scale applications, and/or to address apparatus volume issues perhaps unique to volume-limited applications.